Method and apparatus for controlling induction motor for compressor

ABSTRACT

DC power is supplied to an induction motor for a compressor by switching ON/OFF in response to a modulated voltage waveform obtained from modulating waves which are used for generating AC power for energizing an induction motor and a carrier wave according to PWM theory. The compressor has a compressing element rotated by the induction motor, and the modulating wave is corrected according to a rotational angle of the compressing element. A sine wave having the same frequency as the modulating wave is added to the modulating wave, and a phase of the sine wave is controlled so that a maximum value of the sine wave corresponds to the compression stage of the compressing element. Thus, a rotation speed of the compressing element is constant in a suction stage and a compression stage.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a compressor constituted by housing aninduction motor and a compression element rotated and driven by a rotorof the induction motor in a same casing, and more particularly, to acontrol method and control apparatus which reduces vibration and noiseof the compressor by controlling the driving torque of the inductionmotor.

BACKGROUND OF THE INVENTION

A compressor control system is known from Japanese Patent Laid-Open No.60-60286/1985. The control system disclosed in this reference provides amotor output torque in synchronism with the change of a load torqueapplied in one revolution of a motor rotor for driving a compressionelement of the compressor. In other words, the control system alwaysdetects the load torque during one revolution of the rotor and changesthe motor output in such a manner as to correspond to this load torque.

Generally, as a rotating position detector of a compressor motor, adetector which detects the rotating position of the rotor by couplingdirectly a gear or an encoder to a spindle of the motor and disposing asensor for detecting pulses at the time of revolution, is known asdescribed in Japanese Patent Laid-Open No. 63-23585/1988.

In accordance with the control system of the compressor having theconstruction described above, the rotating position of the compressormust always be detected, and there remain problems in that a largenumber of position detectors are necessary, in that a high level ofaccuracy is required for this position detection, in that the outputs ofthe motor must be successively calculated and outputted sequentially,with the result being the control circuit becomes complicated.

In the rotating position detector of the motor having the constructiondescribed above, the sensor must be disposed directly inside thecompressor and for this reason, the Freon resistance, temperatureresistance requirements and pressure resistance of the sensor must befulfilled and the service life of the sensor cannot be securedsufficiently. Though a sensor having an improved Freon resistance hasbeen developed, a sufficiently long service life cannot yet be secured.

SUMMARY OF THE INVENTION

In view of the problems described above, the present invention isdirected to provide a control method which can effect torque control ofan induction motor by simple position detection and a simple controlsystem.

It is another object of the present invention to provide a controlapparatus of an induction motor for a compressor which does not causethe problems of the Freon resistance, the temperature resistance and thepressure resistance even when using an ordinary general-purpose typesensor.

According to the compressor of the present invention wherein aninduction motor and a compression element rotated and driven by a rotorof the induction motor are housed in a same casing, an A.C. power of apattern having a predetermined change, which is determined in advance soas to correspond to the rotating angle of the compression element, issequentially supplied to the induction motor from a specific phaseposition of the A.C. power whenever the rotating angle of thecompression element reaches a predetermined angle.

The pattern is set so that the frequency at the phase positioncorresponding to the rotating angle, at which the driving torquerequired by the compression element becomes great, is greater than thefrequency of one period of the pattern. Furthermore, the pattern is setso that the change of the frequency in one period of the A.C. powersupplied to the induction motor becomes continuous.

The compressor which employs the control method of the present inventioncomprises typical permanent magnets which rotate with a rotating shaft,recesses formed by depressing part of an enclosed casing so as to facethe permanent magnets, magnetic detection elements in the recesses fordetecting the magnetism of the permanent magnets and a control portionfor determining the rotating position of a rotor from the output changeof the magnetic detection elements. A closed-end cylinder can be usedand inserted from outside the casing in an air-tight manner in place offorming the recess on the casing.

By the control system described above, the A.C. power of the patternwhich changes the output to the induction motor in accordance with thechange of the load torque changing in one revolution of the compressionelement of the compressor can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevation view of a compressor using a controlsystem of the present invention;

FIG. 2 is a sectional view of the compressor taken along line II--IIshown in FIG. 1;

FIG. 3 is a sectional view of the principal portions showing a positiondetector fitted to the compressor shown in FIG. 1;

FIG. 4 is a circuit diagram showing a control circuit for the compressorshown in FIG. 1;

FIG. 5 is a diagram showing ON/OFF activation of the switching devices;

FIG. 6 is a voltage waveform diagram of a three-phase alternatingcurrent obtained by ON/OFF activation of the switching devices;

FIG. 7 is a vibration wafeform diagram obtained by measuring an actualvibration state of the compressor shown in FIG. 1;

FIG. 8 (A), 8 (B), and 8 (C) are flowcharts showing the generation ofthe ON/OFF signals;

FIG. 9 is a diagram showing the relation between MP, SP and DP at thetime of shift of the ON/OFF signal from MP to SP;

FIG. 10 is a waveform diagram of the three-phase alternating currentwhen the three-phase alternating current is supplied to the inductionmotor by use of the flowcharts of FIG. 8A to 8C;

FIG. 11 is a vibration waveform diagram obtained by the control methodof the present invention;

FIG. 12 is a sectional view of the compressor in accordance with anotherembodiment of the present invention and shows a Hall device as anexample of a magnetic detector;

FIG. 13 is an upper sectional view of the compressor shown in FIG. 12;

FIG. 14 is a top plan view of a disc adaptable to a rotor of thecompressor shown in FIGS. 3 and 12;

FIG. 14A is a top plan view showing a modified example of the disc shownin FIG. 14;

FIGS. 15 and 16 are upper sectional views, each showing another exampleof the disc; and

FIG. 17 is a sectional view showing another modified structure of thecompressor.

PREFERRED EMBODIMENTS OF THE INVENTION

Hereinafter, preferred embodiments of the present invention will beexplained with reference to the drawings. FIG. 1 is a sectional view ofa compressor. A sealed casing 1 houses therein a compression element 4and a three-phase induction motor 3. The three-phase induction motor 3has a stator 5 on which three-phase windings are wound and a rotor 6which rotates in response to the magnetic field generated from thestator 5. The compression element 4 is connected to the shaft 13 of therotor 6. The compression element 4 has a shaft 13 and crank portion 14rotating as a crank shaft 2, a roller 8 which is rotated by the crankportion 14 inside a cylinder 7, upper and lower bearing portions 10 and11, respectively, which close the openings of the cylinder 7 and a cupmuffler 12 which is fitted to the upper bearing portion 10. A vane 9 isdisposed in contact with the roller 8 and partitions the inside of thecylinder 7 into a high pressure chamber and a low pressure chamber.Reference numerals 16 and 17 represent balancers, which are disclosed insuch a manner as to balance either dynamically or statically with thecrank portion 14 of the crank shaft 2. A discharge pipe 18 is fitted tothe upper wall of the sealed casing 1.

In FIG. 2, which is a sectional view taken along line II--II of thecompressor shown in FIG. 1, a discharge port 19 and a suction port 20are provided. The zones 21a and 21b, inside the cylinder 7 represent thehigh pressure chamber and the low pressure chamber, respectively. Theroller 8 rotates in a direction represented by the solid line arrows inFIG. 2 and the contact position of the roller 8 with the cylinder 7 isexpressed as a rotating angle up to 360°. In the state shown in FIG. 2,the rotating position of the roller 8, that is, the crank shaft 2 (shaft13), is at the position of 180°.

In FIG. 3, which shows the state where a position detector for detectingthe rotating position of the compression element 4 is fitted to thecompressor shown in FIG. 1, the compression element 4 rotatessimultaneously with the rotor 6 through the crank shaft 2 (the shaft 13of the rotor 6) and, accordingly, the rotating angle of the compressionelement 4 can be detected by simply detecting the rotating angle of therotor 6. The position detector has a magnet 23 bonded to a disc 22 and amagnetic detector (Hall device, Hall IC, coil for magnetic detection,magnetic wire) such as a Hall device 24. The disc 22 is fitted to theshaft 13 by a bolt 25 so that the center of its rotation is in agreementwith that of the shaft 13. The positional relation between the magnet 23and the Hall device 24 is determined such that when the rotating angleshown in FIG. 2 is 0°, the Hall device 24 the detects magnetism andchanges the output. Accordingly, an output can be obtained whenever thecrank portion (rotor) of the compression element 4 has a rotating angleof 0° . The Hall device 24 is disposed at the bottom of a pipe 70 byfitting this closed-end pipe 70 (cylindrical or polygonal) from the sideof the casing 1 of the compressor. The pipe 70 is welded to the casing 1to prevent the leakage of a high pressure gas inside the compressor. Abalancer 26 is provided to correct the weight unbalance occurring whenthe magnet 23 is fitted.

The pipe 70 has a diameter permitting the insertion of the Hall device24, for example, and is made of copper, and a cap is welded to one ofthe ends of the pipe 70 to form the closed-end pipe 70. As will bedescribed presently with reference to FIG. 12A, the Hall device can beconstituted in such a manner that after it is fitted to a substratehaving a size that can be fitted into the pipe 70, the Hall device isthen fitted into the pipe 70 with the substrate.

In FIG. 4, which is a control circuit diagram for controlling theoperation of the compressor shown in FIGS. 1-3, reference numerals 27-29represent the stator windings of the induction motor 5 and thesewindings are star-connected. The magnet 23 fitted to the disc 22 of therotor 5 rotates when a three-phase alternating current is supplied tothe stator windings 27-29. Accordingly, the output of the Hall device 24changes due to the rotation of the rotor 6 (compression element) and theposition detection circuit 30 converts the output change and supplies itto a control portion 31. Switching devices 32-37 effect ON/OFFoperations and are connected in a three-phase bridge form. The switchingdevices 32-37 convert the D.C. power supplied form a D.C. power source39 to the three-phase A.C. power and supplies the three-phase A.C. powerto the three-phase stator windings 27-29. Diodes for discharging thestored charge and for forming a circulation circuit of a circulatingcurrent occurring in the stator windings 27-29 are connected to theswitching devices (semiconductor switching devices such as transistordevices, FET devices, GTO devices, etc) 32-37, respectively. The D.C.power source 39 may be realized either by rectifying and smoothing A.C.power or by a D.C. battery. The ON/OFF operation of the switchingdevices 32-37 is controlled by the signal supplied from the controlportion 31 through a base drive circuit 38. The control portion 31consists primarily of CPU, RAM, ROM, I/O interface, and the like,calculates the frequency on the basis of a speed signal from a speedinstruction circuit 40 and controls the ON/OFF operation of theswitching devices 32-37 so that the A.C. power of this frequency can besupplied to the stator windings 27-29.

Hereinafter, the switching signals to be supplied to the switchingdevices 32-37 will be explained. FIG. 5 is a diagram for obtaining theswitching signals on the basis of the PWM theory. In this diagram,reference numerals 50, 51 and 52 represent sine waves, and their phasesare deviated by 120° from one another. Reference numeral 53 representstriangular waves, and the outputs obtained by comparing the sine waves50-52 with the triangular waves 53 are the switching signals to theswitching devices 32, 34, 36. The switching signals to the switchingdevices 33, 35, 37 are those which are obtained by inverting theswitching signals of the switching devices 32, 34, 36. The three-phasepower having the same frequency as the sine waves 50-52 can be suppliedto the stator windings 27-29 of the induction motor by use of these sixkinds of switching signals. Accordingly, if the frequency of the sinewaves 50-52 is changed, the frequency of the A.C. power to be suppliedto the stator windings 27-29 can be changed. The voltage of thealternating current to be supplied to the stator windings 27-29 (thevoltage when replaced by the sine wave) can be changed by changing theamplitude ratio between the sine waves 50-52 and triangular wave 53. Inthe present invention, the switching signals based on the PAM theory canbe used likewise.

The switching patterns (ON/OFF) of the switching devices 32-37 obtainedin this manner are stored in ROM, all the patterns or part of thepatterns may be stored and may be combined at the time of read-out.Furthermore, the patterns may be stored by dividing the time into thetime for the ON-signal output and the time for the OFF-signal output.Still further, the patterns may be stored by dividing the patterncorresponding to one period into predetermined phase angles and dividingfurther the predetermined phase angle into the ON time and the OFF time.

The switching signals of the switching devices 32-37 may be calculatedfrom the difference between the sine wave and the triangular wave at thetime of a designated phase angle. In this case, the switching signal forone period can be obtained by changing the designated phase angle from0° to 360°.

FIG. 6 is a diagram of the three-phase A.C. voltage waveforms (replacedequivalently to the sine waves) 61-63 obtained by the ON/OFF operationof the switching devices 32-37. The phases of voltage waveforms aredeviated by 120° from one another. In the present control system, thisoutput voltage is not applied merely to the stator windings 27-29 butthe supply of the voltage to the stator windings 27-29 is started fromthe position of the phase angle 0° shown in FIG. 6 in synchronism withthe signal from the position detector whenever the rotating angle of thecrank portion 14 of the compression element 4 reaches zero (0). In thefollowing description, the slip of the motor will be omitted.Accordingly, when the rotating angle of the compression element is 0°,the voltage supplied to the stator windings is always equal to thevoltage at the time of the phase angle =0° shown in FIG. 6. Referencenumeral 67 represents the waveform illustrating the relation of thedriving torque necessary when the compression element 4 is driven. Inthis manner, the torque change exists during one revolution of thecompression element and this torque change results in vibration of thecompressor. Accordingly, the output of the induction motor must beincreased to correspond to this driving torque. Assuming, for example,that the portion requiring a large torque during one revolution of thecompression element is limited near to the portion where the rotatingangle of the crank portion is 200° (in the case of one cylinder), thenthe voltage to be applied to the stator windings may be made higher thanthe ordinary voltage when the crank portions exists at a rotating anglenear to this rotating angle. The rotating angle of 0° of the crankportion 14 and the phase angle of 0° of the voltage pattern are broughtinto conformity by the output of the position detector 30 and therotating angle of the crank portion and the phase angle of the patternof the voltage supplied to the stator windings are in agreement witheach other (with the proviso that the slip of the induction motor isneglected). Therefore, it is possible to cope with this torque change bymaking the voltage higher near the phase angle 200° of the voltagewaveforms 61-63 shown in FIG. 6 than the ordinary voltage.

The voltage waveforms 64-66 shown in FIG. 6 take this torque change intoconsideration. In other words, the voltage waveform 64-66 increases thevoltage and the frequency (the frequency in one period of this pattern,that is, the angular velocity of the phase) near the phase angle of 200°so as to increase the output torque of the motor, and reduces thevoltage and the frequency near the phase angle of 0° so as to reduce theoutput torque of the motor. The voltage and frequency of the voltagepattern are set so that the output changes continuously between thephase angle for increasing motor output and the phase angle fordecreasing it. In FIG. 6, reference numerals 68 and 69 representexamples of the waveform used for voltage correction and the waveformused for frequency correction, respectively. The voltage waveforms 64-68are obtained by multiplying the three-phase A.C. waveforms 61-63 by thecorrection values such as the waveforms 68, 69.

Accordingly, the data for obtaining the switching signals which make itpossible to supply these voltage waveforms 64-66 to the stator windings27-29 are stored in ROM of the control portion 31 as described above.Practically, however, the amplitudes of the voltage waveforms 64-66 areset in accordance with the actual change of the driving torque of thecompression element in order to further change the amplitude of thevoltage waveform in accordance with the frequency.

When these switching signals obtained in this manner are supplied to theswitching devices 32-37, the vibration of the compressor can berestricted by increasing the output of the induction motor 3 when thecompression element 4 requires a large torque.

The switching signals for obtaining the voltage waveforms 61-63 shown inFIG. 6 can be calculated if the frequency f, the output voltage V andthe phase Ph for obtaining the desired switching signal are determined.Incidentally, the frequency f is equal to the frequency signal givenfrom the speed instruction circuit 40 and the basic value of the outputvoltage V is determined on the basis of the value f so as to satisfy therelation V/f=constant, and this constant value is set so that operationefficiency of the induction motor or in other words, the compressor, isdesirable at each frequency. The switching signals for obtaining theA.C. power for one period are supplied by changing the Ph value withinthe range of 0° to 360°. Practically, the value of Ph is advanced by ΔPhperiod, the same switching signal is maintained. If this advancequantity ΔPh is increased, resolution in one period becomes worse and ifthis value is decreased, resolution can be improved, but the optimum ΔPhvalue is set in advance for each frequency from the relationship betweenthe responding switching time of the switching device and the processingcapacity of the control portion 31.

FIG. 7 is a vibration waveform diagram obtained by measuring the actualvibration state of the compressor. Measurement of the vibration waveform71 was made by fitting an acceleration sensor (not shown) to the outerperiphery of the compressor (near the upper end of the stator 5 shown inFIG. 1) and obtaining the waveform from the output of the sensor. Theangles in this diagram are in agreement with the rotating positions ofthe crank portion 14 shown in FIG. 2. As can be seen from this diagram,the amplitude of the vibration becomes great when the crank portion 14of the compression element is located in the range of 130°-270°. Thisrange 130°-270° is in agreement with the compression stroke in thecompression element.

The vibration waveform 71 was obtained when the A.C. power obtained byuse of the voltage waveform 73 which was applied to the stator windingsof the compressor is supplied. A description of the other phases of thethree-phases will be omitted for simplification because only the phaseis different. This voltage waveform 73 can be expressed by the followingformula:

    V=V.sub.0 sin θ                                      (1)

Accordingly, the formula (1) may be corrected so that the driving torqueis increased in the region between 130° and 270°. In other words, theamplitude of the formula (1) in this region is increased so as toincrease the driving torque.

The following example represents the case where this correction is madeby approximation of the sine wave. Since the region in which theincrease in the driving torque is necessary is from 130° to 270°, a sinewave 72 whose peak exists at the center of this region, that is, at theposition of 200°, is set. This sine wave 72 has the same period(frequency) as a sine wave 73 and its function can be expressed asbelow:

    V=A sin (θ=110°)                              (2)

Here, the symbol A represents the amplitude of the sine wave 52.Accordingly, the sine wave for obtaining the required driving torque forthe compression element is the sum of the formulas (1) and (2) and isexpressed by the following formula (3): ##EQU1## The value A/V₀ isselected arbitrarily within the range of 0.05-0.2 in accordance with thefrequency applied to the induction motor.

A large driving torque can be obtained during the compression stroke ofthe compression element by obtaining the switching signals from the sinewave represented by the formula (3) and from the triangular wave 53shown in FIG. 5. At this time, the voltage applied to the inductionmotor during one revolution of the compressor (i.e., the effectivevoltage after being converted to the sine wave) changes. Accordingly,the condition, V/f=constant, described above is no longer satisfied andoperation efficiency of the compressor drops. The drop of efficiency canbe prevented by changing the f value. The f value, that is, thefrequency, is changed in accordance with the change of the voltageduring one revolution of the compressor rotor. The change of thefrequency, too, is corrected so that the peak of correction exists atthe angle of 200°, in the same way as in the correction of the voltage.At this time, correction is made so that correction on the positive sidebecomes greater than correction on the negative side.

This example effect correction by approximation of the sine wave, butthis method is not particularly so limitative. For example, onapproximation formula such as a (sin)² wave, which is used in place ofthe formula (1) in this case, may be set in accordance with the torquecharacteristics of the compression element.

FIG. 8A is a flowchart showing the operation when correction is made byuse of the formula (3).

Initialization is performed at step S1. In other words, themicrocomputer is switched on and initial constants, the constants f=20Hz, Δθ=α, T₁ =β to be used for the starting frequency 20 Hz and aconstant T₂ =τ are set. The time T₂ is a dead time used in the processin which the ON/OFF combination of the switching devices changes. Thistime is set to be longer than the discharge time of the stored charge ofthe switching devices and prevents their short-circuit. Therefore, thistime T₂ depends on the switching speed of the switching device and isgenerally from several hundreds of milli-seconds to several hundreds ofmicro-seconds. In this example, T=0.01 msec.

Whether or not the input of the signal which turns the operation of thecompressor from ON to OFF ("ON→OFF") exists is judged at step S2 and ifsuch an "ON→OFF" signal exists, the process proceeds to step S3, wherethe stop processing of the compressor is executed by sequentiallylowering the frequency of the compressor to thereby stop the compressor.Next, initial setting of constants is made in the same way as at stepS4. Thereafter, the process returns to R.

Whether or not the input of the signal which turns the operation of thecompressor from OFF to ON ("OFF→ON"), or otherwise the operation by auser, exists is judged at step S5 and if such an "OFF→ON" signal exists,the process shifts to step S17, that is, a subroutine SUB1, which willappear later, for starting.

At step S6, a judgement of whether the "F signal exists" is made, thatis, whether or not the signal for setting (or changing) the frequency isreceived. If such an F signal exists, this value is stored in the memoryportion at step S7.

At step S8, a judgement of whether the "position signal exists" is made,that is, whether or not the signal from the magnetic detector isreceived after the compression element reaches a predetermined rotatingangle, and if such a position signal exists, the process proceeds tostep S18, that is, a subroutine SUB1 (which will appear later) for phasealignment.

At step S9, whether or not the timer T₁ achieves time-UP is judged andif the timer T₁ does not achieves time-UP, the process shifts to R. Inother words, steps S2, S5, S6, S8 and S9 are repeated until the timer T1achieves time-UP.

The ON/OFF signal DP for the dead time is supplied at step S10 and atthe same time, the timer T₂ is started. This ON/OFF signal DP representsthe ON/OFF combination state of each switching device and assuming thatsix switching devices exist (if the invertor circuit has three-phases),this signal DP is expressed as follows:

    DP="1, 0, 0, 0, 0, 0, 0, 0"

The actually effective bits are the higher order six bits, and "1"corresponds to ON and "0" to OFF. Since the size of the memory portionhas the 8-bit unit, the lower order two bits are handled as inefficientdata. This ON/OFF signal DP is obtained by calculation at step S15 andis stored in the memory portion DP.

When the ON/OFF signal DP is supplied, an RS-FF (reset/set flip-flop)circuit (not shown) holds the switching device under this ON/OFF stateuntil the next ON/OFF signal is received.

Whether or not the timer T₂ achieves time-up is judged at step S11. Whenthis timer T₂ achieves time-up, the process shifts to step S12.

At step S12, the ON/OFF signal MP is supplied to change the ON/OFF stateof the switching device and at the same time, the timer T₁ is started.The ON/OFF signal MP is obtained by calculation at step S14.

Processing of "θ=θ+Δθ", i.e., processing for advancing the phase angle,is made at step S13 and the phase angle of the alternating current to besupplied to the induction motor, that is, the phase angle of themodulation wave when obtaining the ON/OFF signal, is advance by Δθ.

The ON/OFF signal SP is determined at step 14. This signal SP isdetermined by F(θ) and obtained by comparing the corrected formula (3)described above with the triangular wave 53 shown in FIG. 5. Therefore,the ON/OFF state of the switching devices 32-37 (FIG. 4) at the phaseangle θ can be determined. The ON/OFF state of the switching devices forone period can be obtained by advancing this phase angle by Δθ from 0°to 360°. This SP value is as follows, for example,

    SP="1, 1, 1, 0, 0, 0, 0, 0"

and the lower order two bits are handled as inefficient data in the sameway as the ON/OFF signal DP.

Step S15 makes "DP=SP×MP" and calculates the ON/OFF signal for the deadtime which is used in the process in which the ON/Off signal MP changesto the ON/OFF signal SP. The ON/OFF signal DP is obtained by calculatingthe logical product (AND) at the bit level between the ON/OFF signal MPand the ON/OFF signal SP.

Assuming that SP="1, 1, 1, 0, 0, 0, 0, 0" and MP="1, 0, 0, 0, 1, 1, 0,0", then DP is DP="1, 0, 0, 0, 0, 0, 0, 0". If this relation is as shownin FIG. 9, the ON/OFF state of the switching devices 32-37 changes asMP→DP→SP. Accordingly, when the operation of the arm of the bridgeconsisting of the switching device 34 and the switching device 35, forexample, is considered, the switching device 34 below the arm becomesON→OFF and the switching device 35 above the arm changes to OFF→ON. Atthis time, even if the switching device 34 below the arm is belated dueto the stored charge and is turned OFF belatedly from the switch signal,the switching devices 34, 35 above and below, respectively, the arm inthe ON/OFF signal DP are simultaneously turned OFF. Accordingly, theswitching device 34 above the arm has not yet been turned ON. In otherwords, the switching devices above and below the arm are prevented fromchanging to the ON state simultaneously.

Step S16 executes "MP=SP" and changes the content of the ON/OFF signalMP to the content of the ON/OFF signal SP. Accordingly, the value ofnext DP and the value of MP which is suppled in succession to the DP arestored in the memory portion.

Thereafter the flow process returns to R and the steps S2, S5, S6 and S8are repeated until the timer T₁ achieves time-UP.

FIG. 8B shows the subroutine for start by step S17. Initialization ofthe constants is executed at step S21. In other words, the constants areset to θ=0, N=0, Δθ=α and T₁ =β. The values α and β are used at the timeof f=20 Hz.

At step S22, the ON/OFF signal MP is calculated from the constants whichare set at step S21. The signal MP is calculated by the same calculationas that of step S14 shown in FIG. 8A. However, the sine wave which isnot corrected is used as the modulation wave.

The ON/OFF signal obtained at step S22 is supplied at step S23 and atthe same time, the timer T₁ is started.

The processing of steps S24-S27 are the same as the processing of stepsS13-S16 shown in FIG. 8A. The modulation wave used at step S25 is thesine wave which is not corrected.

Whether or not the timer T₁ achieves time-UP is judged at step S28 andwhen the timer T₁ makes time-UP, the flow shifts to step S29.

At step 29, the ON/OFF signal DP obtained at step S26 is supplied and atthe same time, the timer T₂ is started.

Whether or not the signal from the magnetic detector 24 exists is judgedat step S32 and steps S24-S31 are repeated during the period in whichthe signal does not exist. Therefore, θ is advanced sequentially by Δθand the continuous A.C. output is obtained until such a signal exists.When the signal exists at step S32, the flow proceeds to step S33, wherethe count value N is incremented by 1 as N=N+1. Next, a judgement of"N≧10" is made at step S34 and if "N≧10" is satisfied, the processproceeds to step S35 and returns to the main routine shown in FIG. 8A.

Accordingly, the alternating current having a predetermined frequency issupplied to the induction motor irrespective of the signals from themagnetic detector 24 until the signals are obtained ten times, forexample, at step S32 or in other words, during the period in which theactuation of the compressor is complete so that the revolution of thecompressor becomes stabilized, and the signals from the magneticdetector can be obtained stably.

FIG. 8C shows the subroutine which represents the operation of step S18to be executed when the magnetic detector 24 detects the position signalin FIG. 8A.

At step S41, whether or not the frequency signal F stored in the memoryportion is greater than the A.C. frequency f which is being supplied atpresent is judged and if F>f, the value of f is increased to f=f+1 atstep S42.

Judgement of whether "F<f" is made at step S43 and if "F<f", the valueof f is decreased to f=f-1 at step S44.

At step S45, the values Δθ and T₁ are set on the basis of the value ofthe frequency f.

At step S46, the value of phase angle θ is set to 0. In other words, thepresent phase angle θ is changed to 0.

Steps S47-S49 execute the same operation as that of steps S14-S49 ofFIG. 8A.

The ON/OFF signal DP obtained at step S48 is supplied at step S50 and atthe same time, the timer T₂ is started. In other words, the ON/OFFsignal is changed from MP to DP or MP→DP before the timer T₁ achievestime-UP and the timer T₂ is started.

At step S51, whether or not the timer T₂ achieves time-UP is judged andwhen it does, the process proceeds to step S52 and returns to the mainroutine shown in the flowchart of FIG. 8A.

In this subroutine, therefore, the time of T₁ and the value of Δθ areset once again on the basis of the value f whenever the position signalis obtained from the position detector 30 (FIG. 4) if any change firstexists in the output frequency. Next, the ON/OFF signal DP, which isnecessary to run from the present ON/OFF signal MP to the ON/OFF signalSP at the time of θ=0, is obtained and after this ON/OFF signal DP issupplied, the process returns to the main routine of FIG. 8A.

FIG. 10 shows the waveforms of the three-phase alternating current whenthe three-phase alternating current is supplied to the induction motorby use of the chart of FIGS. 8A to 8C. This waveform diagram shows theoutput waveform in a more comprehensible way by substitutingequivalently the output waveform PWM by the sine wave.

First of all, when the operation is started, the 20 Hz three-phasealternating current devoid of correction is applied in accordance withthe sub-routine SUB1. The induction motor is actuated by thisalternating current. When the induction motor rotates, the permanentmagnet rotates simultaneously and the position signal can be obtainedonce per revolution of the magnet 23 (FIG. 4). The number of thisposition signal is counted and when this count value reaches 10, theprocessing returns to the main routine for the normal operation and thesupply of the corrected three-phase alternating current is started. Thevalue Δθ is increased sequentially and the three-phase alternatingcurrent from 0° to 360° is supplied. Theoretically, the induction motorrotates once at the phase of 360° and the next signal can be obtained,but in practice, the next signal is obtained at the phase of "360°+a"due to the slip of the induction motor. The output of the three-phasealternating current is started once again from the phase 0° at the timeat which this signal is obtained. Thereafter, the phase is returned to0° whenever the signal is obtained and the output of the three-phasealternating current is started.

In this flow chart, the ON/OFF signal is calculated per each cycle, butit is possible to calculate in advance this signal, to store the dataproviding this calculating signal in ROM and to supply continuously thesignals for one period in match with the 0° signal.

FIG. 11 is a vibration waveform diagram showing the change of thevibration waveform when the torque control is made in accordance withthe flowchart described above, where the measurement of vibration ismade in the same way as in FIG. 7. In this diagram, the outputcalculated by the waveformed diagram 61 shown in FIG. 6 is supplied tothe induction motor in the region not having the torque control and theoutput calculated by the waveform diagram 64 shown in FIG. 6 is suppliedto the induction motor in the region having the torque control. As canbe understood from this diagram, the vibration becomes smaller when thetorque control is made. A transient region develops before the vibrationbecomes small enough because the existence and absence of the control ischanged over during the operation of the compressor.

As described above, the control system for the compressor having theinduction motor in the present invention supplies sequentially the A.C.power of the pattern which is determined in advance to correspond to therotating angle of the compression element, to the induction motor fromthe specific phase position of the A.C. power whenever the rotatingangle of the compression element reaches a predetermined angle, and thepattern is formed by increasing the voltage or current at the phaseposition corresponding to the rotating angle at which the driving torquerequired by the compression element becomes great. Accordingly, thevibration resulting from the difference between the driving torque andthe output can be reduced by bringing the output of the induction motorinto conformity with the increase in the driving torque of thecompressor.

FIGS. 12 and 13 are a sectional view of the principal portions and a topview, respectively, and show another example of the rotating positiondetector, and FIG. 14 is a top view of the disc 22 which is common tothe embodiment shown in FIG. 3. The difference from the embodiment ofFIG. 3 resides in that the closed-end pipe 41 is inserted from the upperpart of the sealed casing 1 from the side of the discharge pipe 18 alongthe shaft 13 of the rotor 6. Reference numeral 42 represents the Halldevice, 43 is the seal terminal of the compressor and 44 is a fittingbolt. A cover made of a resin is fitted in such a manner as to cover theseal terminals and the opening of the pipe 41. Since the pipe 41 iscovered with the cover, it is possible to prevent dust and rain waterfrom entering this pipe 41.

In FIG. 14A, showing a modified example of the disc 22, referencenumeral 80 represents a hole through which the bolt 25 is inserted andthe step portion of the bolt 25 clamps this hole and fixes the disc 22.A concavo-convexity may be formed in the hole to prevent an idlerotation of the bolt and the disc. Therefore, this disc 22 rotates withthe rotor. Openings 81 and 82 facilitate the passing of a compressioncooling medium through the disc 22. A fixing member (magnet fixingholder) 83 of the permanent magnet 23 is made of a non-magneticmaterial. The bar ring of the holder 83 is inserted into the caulked tothe hole of the disc 22 at caulking portions 84, 85 when the holder 83is fixed to the disc 22. A through-hole 86 is disposed so that part ofthe permanent magnet 23 can be seen through it when the permanent magnet23 is fixed by the holder 83. When the permanent magnet 23 is seenthrough this through-hole 86, marks such as the polarity indicationdisplayed on the magnet can be confirmed and the mistake of polarity ofthe permanent magnet 23 can be prevented.

FIGS. 15 and 16 are top plan views showing the other embodiments inwhich the permanent magnet is fitted to the disc. FIG. 15 shows the casewhere the permanent magnets 45, 46 are disposed at positions where theysupply the signals when the rotating positions of the rotor are 130° and270°. When the permanent magnets are fitted in this manner, the outputvoltage of the A.C. power may be changed in accordance with the changeof the output of the Hall device 24, 42 and the calculation of therotating position becomes unnecessary. Thus, the controller can besimplified.

FIG. 16 shows the case where the permanent magnets are fitted at thepositions where the rotating positions of the rotor are 0° and 180°. Ifthe permanent magnets are fitted to such positions, the accuraterotating positions can be calculated by correcting twice the rotatingpositions of the rotor during one revolution.

In the compressor shown in FIG. 17, a pipe 70 made of copper has a sizewhich can accept therein the Hall device 24. A cap is welded to one ofthe ends of the pipe to form the closed-end pipe. The Hall device 24 isfitted to the tip of a substrate 92 which has a size capable of beinginserted into the pipe 70, and is then inserted into the pipe 70 withthis substrate. A cover 90 is fitted to the upper part of the casing 1through a gasket 91 by fitting a bolt 93 to a screw hole 96 of thecasing 1. A support portion 95 for the substrate 92 is formed integrallywith the gasket 91. The support portion 95 pushes the substrate 92 tothe bottom of the pipe 70 and at the same time, covers the opening ofthe pipe 70. The support portion 95 and the gasket 91 having an innerdiameter fitting to the outer diameter of the pipe 70 are made of aflexible material, such as a synthetic resin or rubber, and pushes thesubstrate 92 by flexibility of this material. In the drawing, referencenumeral 94 represents an accumulator fitting metal.

We claim:
 1. A method of supplying electric power to an induction motorfor a compressor, said compressor having a compressing element rotatedby said induction motor, comprising the steps of:(1) detecting arotational position of said compressing element, (2) supplying anelectric signal when said detected rotational position corresponds to apredetermined position, (3) controlling the phase of a sine wave so thatthe value of said sine wave becomes maximum after a first predeterminedperiod of time from the supply of said electric signal, (4) setting saidfirst predetermined period of time so that said sine wave becomesmaximum when said compressing element is rotated and positioned in acompression stage, (5) adding said sine wave to modulating waves to formcombined modulating waves, said sine wave having the same frequency assaid modulating waves, (6) generating modulated voltage waveforms fromsaid combined modulating waves and a carrier wave according to PWMtheory, and (7) switching DC power in accordance with said modulatedvoltage waveforms to thereby obtain said electric power.
 2. A method ofsupplying electric power according to claim 1, further comprising thesteps of:(a) storing a switching signal for switching said DC power inaccordance with said modulated voltage waveforms, said switching signalbeing divided into plural data and stored in a memory device, (b)providing outputs in response to said electric signal in a predeterminedorder,wherein said switching signal is generated by the use of saidoutput data, and said DC power is switched to thereby obtain saidelectric power.
 3. An apparatus for controlling an induction motor for acompressor comprising:a casing; an induction motor in said casing; acompressor compression element rotatable in said casing and driven bysaid induction motor by a supply of AC power; a position detector forsupplying a signal when said compression element reaches a predeterminedrotation angle; AC power generation means for sequentially supplying apseudo-sine wave alternating current generated on the basis of pulsewidth modulation in accordance with the change of a phase angle; and anoutput control portion of sequentially supplying said pseudo-sine wavealternating current from a predetermined phase angle to said inductionmotor whenever the pseudo-sine wave alternating current responds to thesignal supplied from said position detector, wherein the modulation waveat the time of pulse width modulation is corrected by use of atriangular function, said triangular function having the same period asthe modulation wave used for pulse width modulation and having a phaseangle which increases in agreement with the rotating angle during acompression stroke of said compression element.
 4. An apparatusaccording to claim 3, wherein said position detector has permanentmagnets disposed inside said casing and rotating with said compressionelement and a Hall device disposed outside said casing to detect themagnetic flux of said permanent magnets.
 5. A method of supplyingelectric DC power to an induction motor for a compressor, saidcompressor having a compressing element rotated by said induction motor,said electric DC power being supplied by switching ON and OFF inresponse to a switching waveform, said switching waveform being obtainedby a modulating wave and a carrier wave according to PWM theory,comprising the steps of:(1) detecting a rotational position of saidcompression element, (2) supplying an electric signal when said detectedrotational position corresponds to a predetermined position, (3)generating a sine wave having a frequency equal to a frequency of saidmodulating wave, (4) regulating a phase of said sine wave when saidelectric signal is supplied so that the value of said sine wave becomesmaximum after a first predetermined period of time from the supply ofsaid electric signal, (5) setting said first predetermined period oftime to be equal to a time period of moving said compressing element toa compression position from the supply of said electric signal, (6)adding said sine wave to said modulating wave to form combinedmodulating waves,wherein said switching signal is generated by the useof said combined modulating waves and said carrier wave according to PWMtheory.